Determination of the composition of samples containing very large molecules is important in many areas of chemistry and biology. Researchers regularly combine gas, liquid, or capillary electrophoresis chromatography with time-of-flight mass spectrometry (C/TOF-MS) to identify and obtain information about the internal structure of the molecules. The chromatography device separates a sample, most commonly, by molecular weight, though those skilled in the art recognize that other physical, chemical or ionic characteristics can also be used to separate the sample. Further, the chromatography device elutes a semi-continuous stream of molecules, with lighter molecules generally eluted first, then gradually heavier ones. This molecular stream is fed directly into a time-of-flight mass spectrometer (TOF-MS) system. In a TOF-MS, the controls for the ionization and fragmentation chamber can be adjusted to inject the unfragmented molecules or to fragment the molecules upon injection into the TOF-MS system. The purpose of the TOF-MS system is to continuously measure the masses of the injected molecules or fragments in order to identify the molecules as they elute from the chromatograph.
Once in the TOF-MS system, the molecules or molecular fragments undergo ionization and are accelerated toward an ion detector by a high voltage pulse. The TOF-MS system determines the mass of the molecules or molecular fragments by measuring the time required for the accelerated ions to travel the fixed distance of the TOF-MS chamber. Ions with the lowest mass and highest electrical charge arrive first at the detector with heavier ions arriving later in time. The time required for the slowest molecule or fragment to travel the entire distance is one detection cycle, also referred to as one record, of the TOF-MS system. In order to improve the accuracy of the measurement, multiple records are acquired in rapid succession and added together to form a mass spectrum. The time required to measure one mass spectrum from a single molecular type eluted from the chromatography device is one "chromatograph sampling interval" of the TOF-MS system. The resulting mass spectrum from the TOF-MS system gives a "fingerprint" which can be used for purposes of identification and structural analysis of the molecules.
In general, there are two popular alternatives for converting the analog signal from the ion detector to a digital record: a) a time digitizer or b) a transient digitizer. With a time digitizer, the ion arrival rate must be limited to a low value so that the time digitizer can measure the arrival time of each ion and convert that time to a digital number. A time digitizer can yield very precise time measurements, but cannot accommodate the very high ion arrival rates required for high sensitivity in a C/TOF-MS instrument.
With a transient digitizer the ion rates can be increased so that many ions of the same mass-to-charge ratio (m/z) arrive at the ion detector simultaneously. The result is an analog voltage pulse whose amplitude is approximately proportional to the number of ions in the pulse. In the transient digitizer, an ADC samples the output waveform of the ion detector and converts the measured analog voltage to a digital representation. The ADC sampling is driven by the edges of a clock pulse so that the detector voltage is sampled periodically. For example, the range of ion flight times from zero to 131 microseconds is typically sampled at two nanosecond intervals. The digital representations of the voltage samples are sequentially stored in a digital memory to form a single record.
Due to the variability caused by ion statistics and the statistics governing the signal gain in the ion detector, one record does not provide an adequate signal-to-noise ratio. Consequently, multiple records must be acquired and summed in rapid succession. A transient digitizer which can perform this rapid summing is known as a digital signal averager.
Although a digital signal averager can process much higher ion rates than a time digitizer, it suffers from worse time resolution. Consequently, this invention focuses on improving the time resolution of the digital signal averager while achieving exceptionally high rates of data collection. For example using the methods disclosed in the present invention, the digital signal averager can acquire and store TOF-MS spectra at a sustained rate of at least 10 spectra per second for a period of at least thirty minutes. Each spectrum in the example is the sum of 189 records and spans flight times from zero to 131 microseconds with data points sampled at 0.5 nanosecond intervals.
Other digital signal averaging devices have been previously disclosed. Typical of the art are those devices disclosed in the following U.S. Patents:
______________________________________ Pat. No. Inventor(s) Issue Date ______________________________________ 5,428,357 Haab et al. Jun. 27, 1995 4,490,806 Enke et al. Dec. 25, 1984 ______________________________________
The U.S. Pat. No. 5,428,357 patent discloses a high speed data acquisition system and method. The device of the '357 patent implements a plurality of data acquisition circuits, typically five, to acquire data at high speed from an analog signal with or without data averaging. Each data acquisition circuit includes all hardware necessary to perform an analog-to-digital conversion, average the digital signal data, and temporarily store the averaged digital signal data prior to transfer to long-term storage. Essentially, the '357 device is a series of independent data acquisition systems operating in parallel. Because each data acquisition circuit employs a single, circular buffer, each memory undergoes one read and one write cycle during each summation limiting the computational speed. Furthermore, the '357 device experiences dead time during measurements because the ADC is disabled while the final summation is read from the memory.
In addition, the '357 device utilizes a lookup table for summing averaged data. The '357 device divides each potential output of the ADC by the number of samples to be taken and stores the quotient in the lookup table. As each sample produces an ADC output, the corresponding quotient in the lookup table is summed to create the averaged signal data.
Because the input sampling rate of the ADC is approximately forty-two times greater than the output transfer rate of each data acquisition circuit of the of the '357 device, non-averaged digital signal data is cached in burst memory. Accordingly, test time is limited by the size of the burst memory. In the '357 device, non-averaging testing times are limited to approximately two seconds.
Finally, when averaging data, sufficient averages must be taken to reduce the effective input rate to a level less than or equal to the output transfer rate for the chosen number of data acquisition circuits.
The U.S. Pat. No. 4,490,806 patent discloses a high repetition rate transient recorder with automatic integration with a maximum sampling rate limited by the maximum rate of available state-of-the-art ADC's. Further the '806 device is limited by the memory architecture which requires an even number of records to be summed, as explained in the following summary of the memory system of '806 device. The '806 device includes three pairs, A, B, and C, of summation devices and memory units for summing and storing digital signal data which is linearly acquired at a constant period. Summation/memory pairs A and C are configured to alternatively work with summation/memory pair B to sum the data for one spectrum. Furthermore, summation/memory pairs A and C are configured to transfer final summation to a permanent storage. In operation, analog data is received from a C/TOF-MS and converted to a digital representation by an ADC. The summation/memory pairs then sum and store the digital records to compile a spectrum, which contains the data from one set of records. The first record of a spectrum is always processed by summation/memory pair B. Within a spectrum, processing of each successive record alternates between either of summation/memory pair A or C and summation/memory pair B. This allows the memory read/write time to be halved by alternatively reading the previous sum from one memory, adding the previous sum to the newly acquired data, and writing the new sum to a second memory. For example, during the second record, the data from the first record is read from summation/memory pair B, summed with the newly acquired data, and the sum stored in summation/memory pair A. During the next record, the sum is stored in summation/memory pair B. This sequence alternates until the required number of records have been summed to form the spectrum. Because the first record is processed by summation/memory pair B and only summation/memory pairs A and C are equipped for output to permanent storage, an even number of records must be acquired for each spectrum so that the final summation is stored in summation memory pair A or C. At the conclusion of each spectrum acquisition, the output is transferred to permanent storage from the output summation/memory pair active during the spectrum acquisition while the other output summation/memory pair works in conjunction to process incoming data. Accordingly, the '806 device is limited to measuring an even number of records, which is undesirable in many applications.
Unfortunately, a C/TOF-MS instrument can produce data at a rate beyond the ability of conventional electronic instruments to analyze. For example, a single molecular type from the sample might be eluted from the chromatography device apparatus as a "surge" of two to five seconds duration which must be sampled in 100 millisecond intervals. The time per record of the TOF-MS system might be 100 microseconds. To obtain all the information available, the summation of one thousand records (100 msec/100 .mu.sec) of the TOF-MS system would be required. The data in each record is obtained by sampling the TOF-MS output by the ADC. Because the fragment peaks in the mass spectrum may be only two nanoseconds wide and several samples are required over each peak, the sampling resolution of the ADC should be at least one sample per 0.5 nanosecond.
For each record, the data is summed with data from previously summed records. The instant example requires that the summing cycle be completed in less than 0.5 nanoseconds for each data point. Because ions arrive continuously, attempts to complete summing during dead time at the end of a pass would result in unacceptable data loss. Further, each pass produces 200,000 data points (100 .mu.sec/0.5 .mu.sec/data point) at the sampling interval of the ADC. After averaging, the resolution of the data increases to a size of eighteen to twenty-four bits, depending upon the ADC resolution. Transfer of all data points to long-term, memory must occur at the conclusion of each chromatograph sampling interval so that a new spectrum can be acquired. For a spectrum having a three-byte data size, approximately 600,000 bytes of data would have to be transferred while incurring less than one millisecond of dead time to achieve less than one percent data loss. One skilled in the art will recognize that readily-available, conventional hardware is not capable of data transfer at this rate.
Furthermore, one skilled in the art will recognize that digital sampling oscilloscopes (DSOs) and fast flash ADC systems solve similar problems in less demanding applications but cannot handle the unique requirements of the C/TOF-MS. By incorporating a variable delay between the trigger and the sampling point, high-speed DSOs achieve high bandwidth with fine time resolution. Each sample can be taken at a small offset relative to the previous sample, however, only one sample is taken for each scan, resulting in low throughput. Accordingly, a large number of scans are required to achieve one complete data record. This implementation is effective for oscilloscopes, but not for data averaging, because the method is prohibitively slow when extensive averaging is required.
Fast flash systems, such as interleaved flash ADCs, incorporate multiple high-speed ADCs operating in parallel. Each sample is processed by a different ADC allowing for a fine time resolution. However, the multiple ADC architecture results in d.c. gain and offset problems which are complicated and expensive to solve when attempting to relate ADC outputs. Furthermore, fast flash systems are not capable of sustained averaging and, therefore, experience significant sampling dead time while samples are being processed. Finally, the large amount of hardware required in fast flash systems is cost prohibitive.
Presently available C/TOF-MS instruments therefore suffer from three practical limitations making them unsuitable for many applications. First, an ADC with an inherent sample rate of at least 2 GHz (0.5 nsec sampling interval) is not presently practical. Second, completing the summing cycle for each data point in less than 0.5 nanoseconds exceeds the capability of any cost-effective, state-of-the-art averaging device. Finally, a data transfer channel with sufficient bandwidth to transfer a mass spectrum to long-term storage without data loss is impracticable.
While both the '806 and the '357 devices process acquired data in parallel, neither device contemplates increasing the effective sampling rate of a conventional ADC. Specifically, the '806 device linearly acquires digital signal data at a fixed phase with respect to the trigger and does not contemplate the use of multiple phase-shifted clock pulses to acquire data in successive phases for later reconstruction. Likewise, the '357 device linearly acquires data at a fixed phase with respect to the trigger.
Accordingly, it is an object of the present invention to provide a high-performance digital signal averager uniquely adapted to an apparatus combining gas, liquid, or capillary electrophoresis chromatography with a time-of-flight mass spectrometer.
Another object is to provide such a digital signal averager capable of meeting the rapid ADC sampling requirements of a C/TOF-MS instrument.
It is a further object of the present invention is to provide such a digital signal averager capable of sustained data processing cycles at high rates without significant data loss.
A still further object of the present invention is to provide such a digital signal averager capable of transferring mass spectrum data to long-term storage at high data transfer rates without data loss.
Yet another object of the present invention is to provide a memory architecture capable of acquiring any number, even or odd, of records in order to overcome deficiencies in the referenced prior art.
Other objects and advantages over the prior art will become apparent to those skilled in the art upon reading the detailed description together with the drawings as described as follows.